Installed new PSU, now have Disk Boot Failure Error...

Hello,
So a while back I had a problem and the good members of this forum help narrow down the problem to a bad PSU and was kind enough to give me a link to what I needed.
I bought the new PSU and installed it into my HP Slimline s5220y and started it up. First thing I noticed was the power light on the Tower doesnt come on, then the next thing i noticed was the "DISK BOOT FAILURE, INSERT SYSTEM DISK AND PRESS ENTER" message.
Looking around the net for an answer, led me to changing settings in my BIO, problem is it goes straight to this error. I cant go into my BIOs.

I have very very important photos from my photography company and want to know if I can keep my files?

Im pretty sure I installed my new PSU correctly, only 4 harnesses used. This is the PSU I bought
Newegg.com - SeaSonic SS-300TFX Bronze 300W TFX12V v2.3 80 PLUS BRONZE Certified Active PFC Power Supply - Power Supplies

Thank you

Our job will be much easier if you could supply as much info - in the System Specs- for the system.

Thanks, I updated it.

I've had a look at the specs for your PC, and, according to that, it originally came with a 220W PSU, so a 300W one should work. Check all connections, particularly the data cables where they plug into the drives and motherboard. If they aren't of the latching type, consider replacing them with latching cables. One of the drawbacks with SATA (and it's the only one) is because the connectors are much smaller, they are far easier to dislodge from their sockets than the old PATA (ribbon cable) connectors. Don't forget to check all other wiring connections as well, as it is very easy to dislodge a wire without realising it, particularly where space is at a premium.

I made sure that everything was in nice and tight. Still same problem. Im not the best at this so maybe i didnt do something right? I will take a picture of how I installed it, maybe it will help.

Here are pictures of my connections.

First is my original PSU


Then my new PSU. Disk drive connection


Hard drive connection


a four pin harness that idk what it does


and the harnesses that i didnt use


Its hard to take a picture of the MB connection but im sure thats fine.

Takaaaa,

First, as already suggested, very carefully check the connectors and their connections.

My suspicion is that one or more cables are not correctly seated/connected.

After inspecting and reseating the connectors, the system still doesn't boot correctly, then does the hard drive make a quiet spinning noise?

You will need to leave the cover off of the computer until the problem is resolved.

Yes, I double checked my connections. Also its hard to hear with the other fans going but i do believe that the hard drive is making a spinning sound

First can you enter your BIOS settings menu? you need to pres a key (usually DEL or F2 but there maybe others please read the manual if you dont know) when pc is booting (before windows boot).. if you can enter bios check if the HDD is detected in bios.
if no then there is problem with HDD or connection.
it would be good to have some kind of botable cd (linux or other bootable testing tools) to check your other hardware.